Added the option 'diary_caption_level', which controls the presence of
captions in the diary index linking to headers within the diary pages.
Possible values:
-1: No headers are read from the diary page.
0: The first header from the diary page is used as the caption.
There are no sub-captions.
1: Captions are created for headers of level 1 in the diary page.
2: Captions are created for headers up to level 2 in the diary page.
etc.
When the value is >= 1, the primary caption of each diary page is set to
the first header read from that page if it is the unique lowest-level
header.
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